The present invention concerns a catalytic composition with improved activity stability with time for the polymerization of propylene. It is known that, in a general way, catalysts based on trialkyl aluminum associated with supported transition metal halides and, more particularly, with trivalent and tetravalent titanium chlorides are not very stable in activity with time in the polymerization of propylene. Contrary to all expectations, it has been observed that isoprenyl aluminum (IPRA), associated with titanium chlorides and, more particularly, with supported TiCl.sub.4, makes it possible to obtain a polymerization catalyst for propylene, which possesses an excellent activity stability with time.
From French Pat. Nos. 2,332,288 and 2,363,583 and U.S. Pat. No. 4,220,745 it is known to associate an organometallic aluminum compound with the catalytic base for the polymerization of olefins, constituted of a titanium halide supported on a magnesium halide and of an electron donor.
Although the activity stability with time of these catalytic systems is recognized in the case of the suspension polymerization of ethylene, it decreases very rapidly in the case of the polymerization of propylene. As already indicated, it has been observed that among the numerous utilizable and utilized organometallic aluminum compounds, only IPRA makes it possible to obtain a catalytic system whose activity increases at the start of polymerization in order to stabilize itself during a long period of polymerization which, depending on conditions, can extend over several hours.